Piezoelectric material is processed into various piezoelectric elements in accordance with different purposes, particularly, it has been widely used for functional electronic components such as an actuator for generating deformation by applying voltage or a sensor for generating voltage from the deformation of elements in a reverse way, etc.
As the piezoelectric material used for an actuator in the disk drive unit for actuating the fine movements of the slider thereof, a lead (Pb)-based dielectric material having large piezoelectric characteristics, especially, Lead Zirconate Titanate Pb(Zr1-xTix)O3-based perovskite-type ferroelectric called as “PZT”, has been widely used, and the piezoelectric material is generally formed by sintering oxide which is formed of individual elements.
Crystal structure of this piezoelectric material formed of PZT varies with the ratio of PbTiO3/PbZrO3. FIG. 1a shows a phase diagram of the PZT. Curie Temperature Tc is a boundary of high-temperature cubic paraelectric phase (Pc) and low-temperature ferroelectric phase. And a morphotropic phase boundary (MPB) divides the ferroelectric phase region into two regions including a tetragonal phase region (FT) and a rhombohedra phase region (FR). As known, when the crystal structure is located at the MPB, the free energy of the spontaneous polarization is quite high, thus this PZT has the best electromechanical conversion property and the best piezoelectric property to obtain an excellent piezoelectric constant d31 or d33.
However, it's quite hard to control the composition exactly located at the MPB. Thus a conventional thin film piezoelectric element often applies the composition near the MPB, such as Pb(Zr0.52Ti0.48)O3 or Pb(Zr0.58Ti0.42)O3. As shown in FIG. 1b, the conventional thin film piezoelectric element 100 includes a substrate 101, two electrode layers 102, 103 formed on the substrate 101, and a piezoelectric layer 104 sandwiched between the two electrode layers 102, 103. The layers 102, 103, 104 are typically deposited by sputtering, laser ablation, Sol-gel coating, and various chemical vapor deposition (CVD) or molecular chemical vapor deposition (MOCVD). Concretely, the substrate 101 is made by Si or other materials such as MgO, etc., and the electrode layers 102, 103 are made by Pt, or conductive oxide SrRuO (SRO), or their combinations, or other conductive materials. Conventionally, the piezoelectric layer 104 includes composition near the MPB, whose crystal structure is tetragonal phase structure or rhombohedra phase structure. And the single-phase piezoelectric layer 104 has a thickness about 2 μm.
However, the piezoelectric constants of this single-phase piezoelectric element 100 is still inadequate as the product requirement for stroke becomes higher and higher. Moreover, the applied field strength is limited due to the single phase structure wherein the depolarization will occur at field strength larger than the coercive field strength (Ec). Conventionally, one way to enable larger applied field strength is to increase the coercive field strength by coercive imprinting which relates to apply a large stress on the piezoelectric element. However, it will bring high risk of delamination and reliability failure for the piezoelectric element. Another way to enable larger applied field strength is to increase the PZT thickness, which will cause a rise in the cost however. Based on the limitation on the piezoelectric constants and coercive field strength, the stroke of the piezoelectric element applied in the actuator or sensor is insufficient, which does not satisfy the requirement of the product manufacturer.
Thus, there is a need for an improved thin film piezoelectric element to overcome the drawbacks mentioned above.